[doc] Document the categories container, remove JSON schema.

[trivialfis]

• Document the category container format.
• Move out the JSON schema to avoid filling the document with a long JSON document.

I haven't included the category container in the JSON schema yet. Looking back, the schema doesn't seem to be particularly useful.

Ref: dmlc/treelite#639

[Copilot]

Pull Request Overview

This PR enhances documentation for XGBoost's JSON serialization format and improves code documentation. The changes focus on explaining the category encoding mechanism introduced in version 3.1 and reorganizing schema documentation.

• Updated documentation comment style in json.h header with stability note
• Added comprehensive documentation for the categories JSON format including feature_segments, sorted_idx, and encoding schemas
• Refactored JSON schema documentation into a separate file with cross-references

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
include/xgboost/json.h	Updated ValueKind enum documentation to use Doxygen format and added stability note
doc/tutorials/saving_model.rst	Added table of contents, new Categories section explaining serialization format, and replaced inline schema with document reference
doc/model_schema.rst	New file created to hold JSON model schema content previously embedded inline

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[hcho3]

Can you update model.schema to include the cats field?

[trivialfis]

Can you update model.schema to include the cats field?

Do you use it?

[hcho3]

No, but other users may decide to use the JSON schema file.

[trivialfis]

but other users may decide to use the JSON schema file.

For context, I'm inclined to remove it.

[hcho3]

Feel free to remove it

[trivialfis]

Removed.